This invention relates to a method of and apparatus for removal of fine particles from a gas stream by high intensity ionization and wet collection.
In order to achieve overall removal efficiencies greater than 95%, it is necessary to remove a major fraction of the fine particles (i.e., below about 3 micrometers diameter) from any particulate laden gas stream. Broadly speaking a wet ionization system is one in which the entrained particulates of a gas stream are electrically charged, forced from the gas stream onto a collecting electrode by an electric field, and removed from the collecting electrode by continuously flowing or intermittently spraying a thin film of wash water thereover. One wet ionization system is the pipe-type electrostatic precipitator, normally used for the collection of moist particulates and corrosive aerosols or in processes with relatively lower flow rates. Emissions with these latter characteristics are often found in the iron and steel, metallurgical chemical, pulp and paper, and mining industries. The emissions of a conventional blast furnace would be one typical example. The present invention pertains specifically to an electrostatic precipitator of the pipe-type.
The two main elements of a typical pipe-type precipitator are a tubular collection electrode, which is normally electrically grounded, and a thin wire discharge electrode, which is connected to a source of high voltage direct current. The wire typically has a diameter between 0.109 and 0.125 inch (0.277 to 0.318 cm), so that it emits a glow discharge or corona when a large potential is imposed between it and the outer tubular collecting electrode. The wire is positioned at the axis of the outer tubular collecting electrode and is typically maintained in this position by a weight which is suspended from the bottom of the wire.
In some pipe-type electrostatic precipitator applications, it is not possible to employ the thin wire design for the discharge electrode. For example, in acid-gas aerosol collection, the discharge electrode is normally formed from lead to ensure a long operating life in the corrosive environment. Because of lead's poor properties in tension, the discharge electrode must be formed with a relatively large overall diameter, for example about 0.375 inch (0.935 cm). However to ensure that a corona discharge is established, the discharge electrode is formed with a non-circular cross-section, for example square or star-shaped, having numerous sites of a small equivalent radius. Functionally, therefore, this design is equivalent to the standard thin wire design.
In operation of the pipe-type electrostatic precipitator, the particulate laden gas is fed into the bottom of the outer tubular collecting electrode which is vertically oriented. The gas passes upwardly through the tube and particulates charged by the corona discharge near the discharge electrode are collected by the outer tubular collecting electrode under the force of the electrostatic field. The clean gas therefore exists from the top end of the outer tubular collecting electrode. In most pipe-type electrostatic precipitators, the collected particulates are removed from the collecting electrode by flowing a thin film of water continuously down its inside wall.
An early design of a pipe-type electrostatic precipitator is described in Conover, U.S. Pat. No. 1,322,163. In the Conover design, the central discharge electrode comprises a metal rod with a series of spaced thin metal discs secured thereto. The surface area of the rod between each of the adjacent thin metal discs is encased within cylindrical separators. The separators are typically formed of an insulating material such as porcelain,, although metallic separators are also disclosed. The separators have a slightly smaller diameter than the thin metal discs. The separators maintain the thin metal discs in spaced position on the rod and prevent any corona leakage from the rod. The periphery of the discs is described as being smoothly contoured and may be semi-circular in shape.
Schwab et al. U.S. Pat. Nos. 4,093,430 and 4,110,806 describe a high intensity ionization system (hereafter referred to as "HII system") for particulate removal from gas streams wherein a disc-shaped discharge electrode is inserted in the throat of a Venturi diffuser. A high D.C. voltage is imposed between the discharge electrode or cathode and the Venturi diffuser, a portion of which acts as an anode. The high voltage between the two electrodes and the particular construction of the cathode disc produces a stable corona discharge of a very high intensity. Particles in the gas which pass through the electrode gap of the Venturi diffuser are charged to very high levels in proportion to their sizes. The entrained particulates are field charged by the strong applied field and by ion impaction in the region of corona discharge between the two electrodes. The high velocity of the gas stream through the Venturi throat prevents the accumulation of space charge within the corona field established at the electrode gap, and thereby improves the stability of the corona discharge between the electrodes.
The prior art has also adapted the Schwab et al. HII system to pipe-type electrostatic precipitators for wet collection. This device includes an inner elongated rod support electrode aligned along the axis of an outer tubular collecting electrode. At least one disc-shaped discharge electrode having a smoothly curved periphery is connected to the rod support electrode and is concentric with the outer tubular collecting electrode. A high potential (D.C.) is applied between the inner rod support electrode-discharge electrode assembly and the outer tubular collecting electrode. Because of the particular design, a thin radially and circumferentially uniform electrostatic field or corona discharge is established between the disc-shaped discharge electrode and the outer tubular collecting electrode, and a non-corona electric field is established between the rod support and the outer tubular collecting electrode. No corona is established between the rod support electrode and the outer tubular collecting electrode because of the relatively large diameter of the rod support electrode.
Particles entrained in the gas which is flowed through this prior art HII pipe-type wet precipitator system are charged as they pass through the corona discharge region established in the gap between the disc-shaped discharge electrode and the outer tubular collecting electrode. These so-charged particles are subsequently collected on the outer tubular collecting electrode under the influence of the non-corona electric field existing in the gap formed between the rod support electrode and the outer tubular collecting electrode. The particles are removed from the outer tubular collecting electrode by a thin film of water flowing along the inner wall of the outer tubular electrode.
According to the prior art HII pipe-type wet precipitator teachings, the disclosed construction is unique in that the electrostatic field between the disc-shaped discharge electrode and the outer tubular collecting electrode is relatively uniform and, therefore, has a high intensity without spark-over. Additionally, the prior practitioners emphasize that the electrostatic field is confined to a small axial direction (no corona in the gap between the rod support electrode and the outer tubular collecting electrode) so that the electrode assembly draws relatively little current when compared to the conventional pipe-type electrostatic precipitator design. Therefore, the power necessary to maintain the high intensity electrostatic field is said to be relatively low. Finally, the relatively large diameter of the rod support electrode with respect to the outer tubular collecting electrode reduces the gap therebetween. This was believed desirable as resulting in a higher strength electric collection field.
A limitation of all prior art pipe-type electrostatic precipitators is limited fine particle removal efficiency. In particular, the wire-type precipitators in current use provide fine particle removal efficiencies on the order of 80%. Although the prior art HII type of wet pipe precipitator represents an improvement by virtue of fine particle removal efficiencies on the order of 90%, a substantial fraction of the fine particles remain in the discharged gas.
An object of this invention is to provide an improved high intensity ionization pipe-type wet collection system.
A further object is to provide an improved HII pipe-type wet collection system for removal of fine particulates from a gas stream, with higher particle removal efficiencies than heretofore achieved.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.